Spectrum analyzer for compressional waves



June 1953 H. HURVITZ SPECTRUM ANALYZER FOR COMFRESSIONAL WAVES FiledJune 21, 1950 AMP.

CONV

AMP.

LIGHT SOURCE IN V EN TOR. "/M

SOURCE OF WAV gs ELECTRICAL Patented June 23, 1953 7 UNITED S'ITATESvPATENT OFFICE SBEQTRUMANALYZER EOR (20M- PRESSIONAL'WAVES;

Ewan Hu hinsi nz. Appflcation- Jllne- 21, 1950, SeriaPNo. roasts whims.(01. 17 183);

The present invention: relates, generally.- to spectrum analyzers, andmore; particularly to spectrum analyzers. for spectra of electricalvi-..

brations and which require-a; minimum of elec-.-.

tronic circuits. for.- accomplishing. the. analysis.

The principal feature. of theinvention is the separation of differentfrequencies in, compres-. sion-al wave bands by virtue; of: the.different propagational velocity whichv the waves expert: ience intraversing prisms. of certainmateria-ls and by virtue of thep p ertiesof gratings.

object of theinvention resides in the pro, vision of a simplified,spectrum analyzer, require i s no devices for. exami n uc essive;portions. of the, spectrum to, determine the presence; of; signals inthe successive portions, as is. true, in. present day panoramic;spectrum, analyzers.

A further object, of theinvention resides in theprovision of a spectrumanalyzer which is capable o analyzing t e u y all. the. requ n ies in apredetermined frequency spectrum.

In accordance with theinvention, a radio spec! um s ct t n lysis isransto medz n o a. spectrum of compressional waves in a mechanical u Tpe ru o eomnr s onal, W v s is directed upon either a body in which thepropagation velocity for the waves varies with; the-frequency of; thewaves, or on someother difierena tial difiracting device, for suchwaves). Such a body then possesses, the characteristics of a prism inrespect to white light. The: body may be immersed in a medium which;presents. substantially. constant propagatiq velocity char, acteristicsfor; all the diiferent. frequency waves under consideration. l?referablythewave props agational velocity in the bqdy'and in the me dium issubstantially equal at. intermediate frequency in the range offrequencies which is; utilized. At higher frequenciesjof that rangethewave velocity in the body mayv exceed that in the medium while at, lowerfrequencies, it. may be, less. quencies, in traversing the body andemergingfrom it into the medium, experience'ref-ractions which aredifferent, andltheidifierent frequencies are accordingly. dispersed" as.are the, different colors of light rays in passing througha disper-.sion prism. The separation of the difierent fre-.

quencies may be accentuated by a. lens which.

focuses them onto a. planearea, at an, angle dee. termined y e.requencyer by a au area exists in which subsistsoompressional' waves ofrelatively high intensity, different positions in the plane areacorresponding with. difierent ire quencies of the waves. made visible bytaking advantage of the fact Accordingly, thewavesof various fre Theseositio sv y h that under the; stresses: produced by compress sionalwayesthe light propagating characteristics of birefringent media.change.

In accordance with a further feature of the invention a diffractiongrating, may be substituted for. the. prism.

invention will be, more readily undere stood-from consideration of thefollowing detailed description taken inconiunction with the accom-.panyin w n wherein:

i re; lustrates; in. p rspective a; p c m analyzer in, accordance Withthe. invention, and empl yi g a r sm, f r: fr qu y s a at n;

F gure. 2 il ustrat s a fra me t ifi ion. hasyst m if-fi ure. 1.. wh a gt emp oyed nstead o a. pr m; and

Figure 3; is a simplified modification of; the sy tem. o 2.-.

Ref r g. t thqd a nss. a so of a spe :v r m Q fre ue cie t e. analy d od a Land a tunable converter 2 for converting these frequencies to afixed band of; frequencies defined by the pass band of an I F.amplifier. These frequenciesbeingfrequencies of electrical vibrations,are applied viaa transmission line 4 to a vibrating crystal 5, whichtranslates the c ic brati'qns into mech ni a vib a ns.

Th cr sta 5 s eme ge a l d. 6 s c as.

water, contained in elongated narrow container; 'I, so that;themechanical vibrations of the crystal 5 generate compressional wavesin the water, which travel as at, 8 toward a refractmg prism 9. I

The refracting prism, may be made of various 35 materials, as is taughtingreater detail in U. S.

Patent No. 2,423,459 issued to W. P. Mason on July 8,1951% As is thereexplained any material which exhibits a different propagational velocityfor compressionalw'aves than does the water through which the beamarrives may be used as arefracting prism, the following additionalproperty being desirable, namely, that the propagational velocity forcompressional waves in the materialvaries with the frequency of thewaves, i ce a ha ma erial. r le t. t d nt frequency compressional wavesdifierently, in a manner, similar to that. in which a dispersion prismdifferently refracts diiferent colors of light.

In the present case, then, the prism 9 may be made of, Lucite,polystyrene or the like, Lucite being a trade name for methylmethacrylate. If we assume, for example, that the band of frequencies tobe analyzed extends from 50 to 20 0 1;. c. at 50 k. c. the velocity ofsound in Lucite will be found to belower than in water, and at 200-11.0., higher, since the velocity of sound in diverging eifects which occurin the beams. vIn.

order to overcome this difficulty a super-sonic lens It is provided,which may be made of Lucite, and which may be designed to have suchcharacteristics as to focus the various beams on a common plane. Eachfrequency, accordingly, finds a different focal point along the plane,and if the focal points can be made visible it follows that thespectrum, or the frequency content of the spectrum, may be visuallyanalyzed.

In order to render the focal points visible, account is taken of the-fact that at these focal points relatively violent compressional wavesexist, while at points slightly separated therefrom such waves exist, ifat all, in highly attenuated form.

Accordingly, a source of light II is provided which projects a strongbeam of lightthrough collimating slits l2, and transversely through thecontainer 1. In the path of the lightbeam is placed in sequence apolarizer l3, a sheet of Pyrex or glass or quartz [4 subject to impactby the compressional-waves, and an analyzer I5.

Polallzer I3 and analyzer l5 may be properly oriented sheets of Polaroidor sheet. polarizing material properly oriented respectively in respectto their planes of polarization. At areas of compression thebirefringence of the sheet of Pyrex, v

diifract the different frequencies of a spectrum.

This grating may comprise a metallic sheet having thin slits cut thereinand may be readily designed to diifract waves of far higher frequency,extending over a far wider band, than the Lucite prism 9.

Reference is now made to Figure 3 of the accompanying drawings wherein-is illustrated a simplified and preferred embodiment of the invention.In the system of Figure 3 the block 20 represents a source of electricalwaves-in a rela tively extended spectrum, which may be derived from aradio frequency receiver, as in the system of Figure 1, or otherwise.The electric waves provided by the source 20 are applied to a crystal 2!which is cemented to or molded within a Lucite block 22, and whichaccordingly creates compressional waves in the block, which travel inthe direction of the arrow, provided in the drawings. Molded within theLucite block 22 is a diffraction grating 23, which may comprisespecifically a sheet of metal having slits therein, the slits having theappropriate widths and spacings to diffract adequately the compressionalwaves traveling along the Lucite block 22. The

Lucite block 22 then diverges, as at 24, to allow for the divergence ofthe compressional waves as they pass beyond the diffraction grating 23.The waves then diverge after passing through the grating 23 in theLucite block section 24, in

4 accordance with their frequencies, so that a separation ofcompressional waves of different frequencies takes place. The Luciteblock section 24 may terminate in a substantially flatplane as at 25.

A polarizer 26 and an analyzer 21, which may comprise narrow thin stripsof Polaroid or like polarizing material, may be cemented to or moldedwithin the Lucite block section 24 adjacent the plane 25, and onopposite sides thereof, so that the compressional waves traveling alongthe Lucite block section 24 pass between the polarizer 26 and theanalyzer 2'1 respectively. A source of light 28 is provided, light fromwhich impinges on the polarizer 26 via suitable collimat- -ing slits-29,passes through the Lucite transversely of the Lucite section 2 4, andthrough the analyzer 27. At those points of the plane 25wherecompressional waves occur, birefringent phenomena exist, so thatthe plane of polarization of the light in passing through the Lucite isvaried; Accordingly, the polarizer 26 and the analyzer 2'? are sospaced,that in the absence of birefringent phenomena no light may be visible onthe analyzer 21, in the presence of such compressional waves light maybe enabled to pass. This light then is visible at a vertical position onthe analyzer 21 which is representative of the frequency of thecompressional wave which enabled its passage.

Accordingly, the species of Figure 3 represents an extremely simplifiedembodiment of the invention, involving no liquids, and which may bemolded in simple manner of Lucite,'or similar plastic material which iscapable of responding to compressional waves, and which has the desiredbirefringent properties, such as Pyrex or glass; If plastic or othermaterial is utilized for the body of the device which does not have thedesired birefringent properties, a slab of Pyrex or glass or Lucite maybe included at the position between the polarizer 26 and analyzer 21. Ifdesired, an absorbent material, suchas copper fillings or screening (notshown) may be utilized to terminate the system by application to theplane 25. By suitably designing the diffraction grating 23 an extremelylarge separation of adjacent frequencies may be attained, by the simpleexpedient of extending the vertical dimension of the plane 25, or theangle of divergence of the Lucite block section 2 By virtue of theconstruction envisaged in Figure 3, there results a portable device, noteasily damaged, not subject to variation of the position of thecomponents in response to' vibration, shock or the like, and ofextremely simple character, which is capable of the simultaneousanalysis of the frequency content of a spectrum.

While I have described and illustrated one specific embodiment of myinvention, it will be clear that variations thereof may be resorted towithout departing from the true spirit and scope of the invention.

What I claim and desire to secure by Letters Patent of the United Statesis:

1. A spectrum analyzer for a band of electrical waves comprising aplurality of unknown frequencies, comprising, means for translating saidband of electrical waves into a congruent band of compressional waves ina birefringent medium, a diffracting device for differently diffractingthe compressional waves of different frequencies into differentpositions of a pre-assigned plane in said medium, and means'forrendering difierentially visible the positions of said pre-assignedplane in said medium which are subject to said compressional waves andthe positions of said pre-assigned plane which are not so subject, saidlast means comprising a polarizer and an analyzer.

2. A spectrum analyzer comprising, a mass of birefringent solid moldablematerials capable of transferring compressional waves, means fordirecting through said mass of solid birefringent moldable material aspectrum of compressional waves of various frequencies, a diffractiongrating located in the patch of said waves within said mass of solidbirefringent moldable material, for differentially diffracting saidwaves in accordance with their frequencies, and means for making visiblethe physical positions of said waves in said material after diffractionof said waves, said last means comprising a polarizer and an analyzer.

3. A spectrum analyzer comprising, a mass of solid birefringent materialcapable of transferring compressional waves, means for directing throughsaid mass a spectrum of compressional waves of various frequencies,means in said mass for diffracting said compressional waves in said massin accordance with the frequencies of said compressional waves, andmeans for making visible the physical locations of the diffractedcompressional waves in said material comprising a polarizer and ananalyzer located on opposite sides of the paths of said waves in saidmaterial.

4. A spectrum analyzer comprising, a mass of birefringent solid moldablematerials capable of transferring compressional waves, means fordirecting into said mass a spectrum of compressional waves of variousfrequencies, means located in the path of said waves for differentiallydiffracting said waves in accordance with their frequencies, and meanscomprising an analyzer and alpolarizer located respectively on oppositesides of said path of said waves in said material for making visible thephysical positions of said waves after diffraction.

5. The combination in accordance with claim 4 wherein said means fordirecting comprises a piezo-electric crystal molded in said moldablematerial.

6. The combination in accordance with claim 4 wherein said means fordiffracting comprises a grating molded in said moldable material.

HYMAN HURVITZ.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,169,304 Tournier Aug. 15, 1939 2,877,242 Kent May 29, 19452,418,964 Arenberg Apr. 15, 1947 2,455,389 Soller Dec. '7, 1948

